1. Field of the Invention
The invention relates to electric lamps. The invention in particular relates to an electric lamp having a glass envelope, the wall of the glass being amber colored.
The invention applies to incandescent lamps and more particularly to automotive signal lamps, e.g. indicator lamps.
2. Description of Related Art
U.S. Pat. No. 5,470,805 describes a lead-free glass composition for use in electric lamps.
It is an object of the invention to provide an electric lamp having a glass envelope, the wall of the glass being amber colored and the glass composition being lead-free.
It is a further object of the invention to provide an electric lamp having a glass envelope, the wall of the glass envelope being amber colored and the glass composition comprising no toxic or forbidden component which could be harmful to the environment such as lead oxide, antimony oxide, selenium or cadmium. In Europe the use of cadmium is restricted to automotive lamps.
According to the invention, these objects are achieved by an electric lamp as mentioned in the opening paragraph the glass composition of the wall comprising up to 1% by weight molybdenum in oxidic form and up to 2.5% by weight SO3.
The electric lamp has a wall of the glass having a lead-free, cadmium-free, sulfate-refined glass composition comprising molybdenum oxide and SO3.
Advantages of the electric lamp according to the invention relate to the following aspects. The glass in prior-art colored lamps, such as automotive signal lamps. usually contains a colored varnish or lacquer. A disadvantage of the prior-art lamps is that the lacquer or varnish may degrade and eventually became detached from the wall of the lamp envelope. Another disadvantage of the prior-art lamps is that additional steps are necessary during the manufacturing process of the lamp to clean and varnish the lamp envelope. The manufacturing cost is increased by these additional steps.
During the production of an electric lamp according to the invention, no additional manufacturing step is necessary since the specific color point of the glass envelope is realized by the particular composition of the glass. At least the steps of cleaning and varnishing are rendered superfluous. Due to its specific composition, the glass may have an amber or orange color point suitable for automotive signal lamps, as defined by international traffic regulations.
Moreover, the colored glass obtained by using this glass composition yields a higher-quality glass, since the amber or orange obtained color due to the glass composition stays forever, which may not be the case of varnished glasses.
The invention and additional features, which may be optionally used to implement the invention, are apparent from and will be elucidated with reference to the embodiments described hereinafter.